The present invention relates to machine tools, such as hand drills and more particularly to control devices utilized for controlling a speed of the tool machine.
It is known in the art that a predetermined rotational speed of the tool machine is preliminarily adjusted. This adjustment particularly takes place in drilling machines which employ a stage-selecting switch which adjusts a tool drive for drilling in the first stage of rotation with a relatively slow rotational speed. When the stage-selecting switch is pushed on it switches the machine drive to a second stage so that the drive is set up to a predetermined speed. A well-known bridging switch was utilized in the conventional tool machines in a working process where drills should be changed one after another for drilling operations in different materials. Such bridging switch, however, was rather inefficient. The above-described adjustment is attained in the known hand tool machines by means of a relatively small regulator. Such type of adjustment is rather difficult because this regulator is not the most favorable in use during switching on and off the machine. Such regulators are particularly undesirable in hand tool machines since the operator's hand must be additionally used for switching this regulator on and off. In these conditions the manipulation of the handle becomes rather difficult and the hand tool machine is not reliably supported. When it is necessary to change a tool utilized in the tool machine of the conventional type the adjusting operation by means of the above-mentioned regulator becomes specifically complicated. In this respect the disadvantages of the conventional devices can not be overcome particularly when frequent and quick changes of tools to be used are required. In such cases the tool drive utilized in tool machines does not appear to be optimal. The adjustment is not complete, rather difficult and time-consuming. It is to be understood that during the drilling operation not only diameters of holes to be drilled but also some other factors such as material to be treated and the drill's material must also be taken into consideration.